docs: def _compute_weight_std(self, C, mag):
          """
          Common part of equations 8 and 9, page 971.
          """
          if mag < 6.0:
              return C['a1']
          elif mag >= 6.0 and mag < 6.5:
              return C['a1'] + (C['a2'] - C['a1']) * ((mag - 6.0) / 0.5)
          else:
              return C['a2']